A cellulose derivative which has a suitable elastic modulus and viscosity when it is dissolved in water and is useful as an adhesion barrier.The cellulose derivative has a molecular weight of 5×103 to 5×106 and is composed of a repeating unit represented by the following formula (1): wherein R1, R2 and R3 are each independently selected from the group consisting of the following formulas (a), (b), (c) and (d) in predetermined amount: (X in the formula (c) is an alkali metal, and R4 and R5 in the formula (d) are each independently an alkyl group or alkenyl group having 10 to 28 carbon atoms).